Oil field operators demand a great quantity of information relating to the parameters and conditions encountered downhole. Such information typically includes characteristics of the earth formations traversed by the borehole, and data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” can be performed by several methods including wireline logging and “logging while drilling” (LWD).
In wireline logging, a probe or “sonde” is lowered into the borehole after some or the entire well has been drilled. The sonde hangs at the end of a long cable or “wireline” that provides mechanical support to the sonde as it traverses the borehole, collecting measurements as a function of position. The wireline may also provide electrical and/or optical conductors for high bandwidth communications between the sonde and the surface electronics.
In LWD, the drilling assembly includes sensing instruments that measure various parameters as the formation is being penetrated. While LWD techniques allow more contemporaneous formation measurements, drilling operations create an environment that is generally hostile to electronic instrumentation and sensor operations. Most telemetry systems designed for this environment are bandwidth restricted, requiring operators to send only the highest-priority measurements (in compressed form) in “real-time” and store the rest for later retrieval, perhaps when the drilling assembly is recovered from the borehole.
In the competition for bandwidth, common casualties are those tools that generate high measurement data rates (e.g., multi-array resistivity tools, magnetic resonance (MR) logging tools, and acoustic waveform logging tools). To provide measurements in a fashion approaching “real-time”, such tools routinely employ significant amounts of downhole processing and compression to condense the raw measurements into a (hopefully) representative bitstream.
More surprising casualties are those sensors that generate low bandwidth measurement streams which are nevertheless get stored (and possibly summarized) rather than being transmitted to the surface in a timely fashion. For example, bandwidth limitations may be sufficiently severe that the engineers omit downhole temperature measurements from the real-time telemetry stream, electing instead to summarize and store such measurements for later retrieval. Intermediate solutions, such as sending a temperature measurement once every hour, are also possible. Despite being driven by necessity, such choices are often considered prudent due to assumptions about the expected behavior of such measurements. Such assumptions may be wrong, often leading to unexpected and “inexplicable” failures of the downhole tools.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.